The present invention relates to an aqueous ink for ink jet printing by use of a dot printer and for use with ball point pens and fountain pens.
Conventionally, there is known an aqueous ink for ink jet printing in which a halogenated xanthene-type dye having the following formula (I) is employed: ##STR3##
Although the above dye has an excellent color tone and high solubility in water and polyhydric alcohols, it has the shortcoming that it fades significantly when exposed to light.
There is also known an aqueous ink for ink jet printing in which another xanthene-type dye having the following formula (II) is employed: ##STR4## wherein R.sup.1 and R.sup.2 each represent ##STR5## (in which R.sup.5, R.sup.6, R.sup.7 and R.sup.8 each represent a lower alkyl group, an amino group, a sulfonic acid group in the form of an alkali metal salt, or a carboxyl group in the form of an alkali metal salt); R.sup.3 and R.sup.4 each represent a lower alkyl group, an amino group, a sulfonic acid group in the form of an alkali metal salt, or a carboxyl group in the form of an alkali metal salt; and k, l, m and n each represent an integer of 0, 1 or 2.
The above dye having the formula (II) does not fade when exposed to light since it has high light resistance, but the color tone is slightly inferior to the first mentioned dye and the solubility in water and polyhydric alcohols is not as high as the first mentioned magenta dye.
Generally, it is preferable that magenta dyes have high spectral absorption peaks in a wavelength range of 500 nm to 600 nm. However, most magenta dyes have also spectral absorption in the base of the absorption curves, for instance, in the ranges of from 400 nm to 500 nm and from 600 nm to 700 nm and because of such spectral absorption, the color tone of such magenta dyes is not good. A magenta dye having a sharp peak in the first mentioned wavelength range, however, is somehow vulnerable to light and fades easily when exposed to light. Therefore it is not suitable for use as a dye of an ink jet printing ink.